The present invention relates to nuclear densometers, and more particularly, to a method and apparatus for processing data to obtain a fast response to changes in the density of a fluid.
The prior art has developed digitally processed data for nuclear densometers, as illustrated by U.S. Pat. No. 3,657,532, issued to Carl W. Zimmerman. As set forth in the '532 Patent, digital systems allow the incorporation of extremely reliable, inexpensive and compact integrated circuits and can be used to develop digital pulse counting techniques. However, in the prior art, there remains a substantially long time response to a change in density in the fluid sample being tested and a consequence, a considerable volume of incorrect density fluid may be passed through the system for use before a correction in the density can be detected or made.
In oilwell cementing operations, the density of the cementing slurry is an important factor. The bore hole cementing fluid typically is a slurry of chemical constituents mixed with water and has a certain density. Should the composition of the slurry mixture change during the pumping operation, the density obviously changes and a change in mixture can affect the desired results in the cementing operation. For that reason, it is desirable to be able to quickly sense changes in density, i.e. changes in the mixture and to be able to provide a correction to the mixture before a large volume of incorrect mixture is introduced into the system.
Similarly, in fracturing of wells, monitoring of the density of the fracturing fluid is desirable.
It is accordingly a feature of the present invention to obtain a relatively quick response time to the change of density in a cementing or fracturing fluid system so that the fluid may be continuously monitored and corrected, if necessary to obtain a consistent density for the fluid mixture.
The present invention involves a method and system for sensing the density of a fluid for providing statistical count signals which are proportional to density and processing the count signals so that significant changes in density produce a quick response and indication of such change and so that statistical variations in the count signals are suppressed.
In one aspect of the present invention count signals are processed by a circuit means which has one or more fixed differential threshold detection levels so that the magnitude of the detected signals must exceed the fixed differential threshold level for a period of time. The circuit means utilizes an RC charging circuit with reversely connected diodes in series with the resistance so that when the forward voltage drops of a diode is exceeded, the capacitor is rapidly charged to produce an output signal. With more than one such circuit the range of response time can be expanded.
In another aspect of the present invention, count signals are processed by a computer to determine a relationship. When the count signals over a time period indicate a significant change in density an indication of such change is developed by using both a large and a small weighting factor. For random changes in density an indication is developed with a small weighting factor normally. The computer processes the count signals to ascertain the magnitude and duration of any changes in the count signal and upon meeting pre-set criteria produces an indication of the change.